To understand the bioaccessibility of the flavonoid quercetin we studied its interaction with bile salt micelles. The environmental sensitivity of quercetin's UV-visible absorption spectrum gave information about quercetin partitioning. Two quercetin absorption peaks gave complementary information: Peak A (240-280nm) on the intermicellar phase and Peak B (340-440nm) on the micellar phase. Thus, by altering pH, we showed that only non-ionised quercetin partitions into micelles. We validated our interpretation by studying quercetin's interaction with SDS micelles. Pyrene fluorescence and the quercetin UV-visible spectra show that the adsorption site for pyrene and quercetin in bile salt micelles is more hydrophobic than that for SDS micelles. Also, both quercetin and pyrene reported a higher critical micelle concentration for bile salts than for SDS. Our method of using a flavonoid as an intrinsic probe, is generally applicable to other lipophilic bioactives, whenever they have observable environmental dependent properties.

Phenolic structures are of great interest due to their antioxidant properties and various postulated benefits on human health. However, the quantification of these structures in fruits and vegetables, as well as in vivo or in vitro experiments, is demanding, as relevant concentrations are often low, causing problems in exactly weighing the respective amounts. Nevertheless, the determination of used concentrations is often a prerequisite for accurate results. A possibility to quantify polyphenol is the use of UV/vis spectroscopy. Therefore, the absorption coefficients of selected phenolic structures were determined in three different solvents relevant for polyphenol research (water/methanol (50/50, v/v), water, and phosphate buffer at pH 7.5). To confirm the values based on weight and to avoid errors due to impurities, hygroscopic effects, and inadequate balance care, the mass concentrations were additionally determined by quantitative NMR (q-NMR). The coefficients presented in this article can help to quickly and easily determine accurate concentrations in a laboratory routine without wasting the often-precious standard compounds.

We have studied the uptake of quercetin aglycone into CaCo-2/TC7 cells in the presence and absence of mixed micelles that are present in the human small intestine. The micelles inhibited the transport of quercetin into the cells. To gain an understanding of why this is the case we examined the solubilisation of quercetin in micelles of differing composition and into pure lipid phases. We did this by using the environmental sensitivity of quercetin’s UV-visible absorption spectra and measurement of free quercetin by filtration of the micellar solutions. The nature of the micelles was also studied by pyrene fluorescence. We found that the partitioning of quercetin into simple bile salt micelles was low and for mixed micelles was inhibited by increasing the bile salt concentration. The affinity of quercetin decreased in the order egg phosphatidylcholine (PC) = lysoPC > mixed micelles > bile salts. These results, together with the innate properties of quercetin, contribute to an understanding of the low bioavailability of quercetin.

Health benefits of apple polyphenols for different chronic diseases are postulated. To exert bioactive properties, absorption into the body is required (bioavailability), which is strongly influenced by matrix release (bioaccessibility). For seven apple varieties, in vitro experiments with simulated saliva fluid (SSF) and ex vivo digestion with centrifuged human saliva were conducted. Polyphenol characterization (high-performance liquid chromatography-tandem mass spectrometry) and quantification (high performance liquid chromatography-diode array detection) was related to an aqueous methanolic extraction. A polyphenol release of 63–82% from flesh and 42–58% from peel was estimated. While hydroxycinnamic acid derivatives were released in total, a significant retention was observed for flavanes and flavones. In particular, procyanidins were retained with increasing molecular weight. The data reveal a considerable polyphenol release during the oral digestion; however, differences among the varieties as well as flesh and peel were obvious. Due to negligible differences between both digestion media, the data supported the use of SSF instead of human saliva in further experiments.

Scope: Apples are an important polyphenol (PP) source. To compare the health benefits of traditional and commercial varieties, the phenolic contents and profiles as well as their release from the matrix (bioaccessibility) during oral digestion are determined. Furthermore, based on these data the proposed beneficial effect of PP on the variety specific allergenicity is discussed.Methods and results: Phenolics are quantified by HPLC‐DAD. Total phenolic contents (TPC) are in the range of 111–645 and 343–1950 mg 100 g⁻¹ dry weight for flesh and peel, respectively. Matrix release during oral digestion is investigated ex vivo, with centrifuged and non‐centrifuged human saliva and in vitro with simulated saliva fluid (SSF). The overall bioaccessibility is similar in all digestion media, ranging between 40–80% and 39–65% of the TPC in flesh and peel, respectively. Analyzing the correlation among Mal‐d 1‐allergen‐content, unoxidized PP, and the allergenic potential for the samples reveals a negligible effect of phenolics.Conclusion: Due to higher phenolic contents in combination with a similar release, increased PP concentrations in the oral phase and an improved uptake of PP from traditional varieties are assumed. However, the proposed beneficial effect of phenolics on allergenicity cannot be confirmed.